diff -r 36f08ae87ef6 -r 13da36d77a3f mercurial/scmutil.py
--- a/mercurial/scmutil.py	Sat Feb 29 12:56:37 2020 +0530
+++ b/mercurial/scmutil.py	Sat Feb 29 12:58:13 2020 +0530
@@ -1900,8 +1900,11 @@
 _reportstroubledchangesets = True
 
 
-def registersummarycallback(repo, otr, txnname=b''):
+def registersummarycallback(repo, otr, txnname=b'', as_validator=False):
     """register a callback to issue a summary after the transaction is closed
+
+    If as_validator is true, then the callbacks are registered as transaction
+    validators instead
     """
 
     def txmatch(sources):
@@ -1927,7 +1930,10 @@
             func(repo, tr)
 
         newcat = b'%02i-txnreport' % len(categories)
-        otr.addpostclose(newcat, wrapped)
+        if as_validator:
+            otr.addvalidator(newcat, wrapped)
+        else:
+            otr.addpostclose(newcat, wrapped)
         categories.append(newcat)
         return wrapped
 
@@ -1942,6 +1948,8 @@
             if cgheads:
                 htext = _(b" (%+d heads)") % cgheads
             msg = _(b"added %d changesets with %d changes to %d files%s\n")
+            if as_validator:
+                msg = _(b"adding %d changesets with %d changes to %d files%s\n")
             assert repo is not None  # help pytype
             repo.ui.status(msg % (cgchangesets, cgrevisions, cgfiles, htext))
 
@@ -1954,7 +1962,10 @@
             if newmarkers:
                 repo.ui.status(_(b'%i new obsolescence markers\n') % newmarkers)
             if obsoleted:
-                repo.ui.status(_(b'obsoleted %i changesets\n') % len(obsoleted))
+                msg = _(b'obsoleted %i changesets\n')
+                if as_validator:
+                    msg = _(b'obsoleting %i changesets\n')
+                repo.ui.status(msg % len(obsoleted))
 
     if obsolete.isenabled(
         repo, obsolete.createmarkersopt
@@ -2057,9 +2068,10 @@
             ]
             if not published:
                 return
-            repo.ui.status(
-                _(b'%d local changesets published\n') % len(published)
-            )
+            msg = _(b'%d local changesets published\n')
+            if as_validator:
+                msg = _(b'%d local changesets will be published\n')
+            repo.ui.status(msg % len(published))
 
 
 def getinstabilitymessage(delta, instability):